The mechanisms behind rock-weathering processes can provide vital clues for understanding and reconstructing the history of ancient environments and visualizing the physical conditions in which they were formed, especially climatic situations. Thick ancient coverings of weathered material such as laterites are still the most intensively studied to date. However, little is known about the initial stages of weathering, owing to the rare occurrence of well-preserved examples.
As a contribution to the PEGI-PROSE (1) programme, scientists from the IRD, the CNRS and the University of Strasburg (2) are conducting investigations on Mount Cameroon. They have identified some of the mechanisms that operate during the first stages of basalt rock weathering and have postulated a particular weathering rate. For that they have studied interactions between water and rock by analysing the chemical compositions of spring waters arising from rainwater percolation through the rock. This rock undergoes changes in response to the various factors (such as temperature, precipitation rate, vegetation) that prevail. The processes give rise to the formation of secondary minerals such as hydrated silicates.
Mount Cameroon, an active volcano which is also the country’s highest peak (4095 m), was chosen for its geological, geographical and climatic characteristics. It has become a reference site for the study of basaltic rocks in a humid tropical climate. Its volcanic nature, massive form and location on the Atlantic coast offers some extreme and varied conditions of temperature and rainfall (3). The parent-rock consists mainly of basaltic lava flows and pyroclasts (pumice, ash). This type weathers 10 to 100 times more rapidly than the other continental rocks owing to its high glass content and the porosity of the pyroclasts, which makes it possible to study the initial stages of weathering. The recent activity of Mount Cameroon has moreover been well mapped (4) and the oldest lavas date from 11 million years B.P. (Upper Miocene).
Marie Guillaume | alfa
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